If you can find at least one other person to help, I'd recommend Sathir's Tomb. It's fairly easy to pull there and the mobs give really good XP. In general T2 RoS gives good XP, but ST is probably the easiest of the zones. It isn't the best zone to solo in, though. It's doable, but your merc will likely run out of mana quickly and the mobs hit a little hard if you end up with multiples. It's definitely possible, but probably not that efficient.

I'm still trying to find a good spot to do a molo lesson burn. The Droga/Chardok HAs are options and are nice because they're instanced, and the sarnak fort in FM gives decent solo XP. T1 RoS zones can be soloed but they give pretty crappy XP, so I wouldn't recommend those. ST (or Gorowyn or w/e) is also an option, it's just not as amazing solo.

For me, for a lesson burn I go to HS or ST. In HS I just drop into the pit and there's single mobs to pull. If you pay attention you will always get singles. There's a VP quest named that spawns so be careful of that if moloing. He's doable with proper AA's and gear, so just be aware.

ST is my favorite zone for moloing and it's even better with a group. XP comes fast. When you first zone into ST you will notice there's a group of 4 mobs spaced perfectly for moloing. There's 2 skeletons. A specter and a goo. Just cycle through them. I have the EM 25 pet, so it makes life easier in the higher end zones.

The biggest problem with ST is getting there. With another player, it's easy. As a mage going there alone, it can be hazardous. First of all, you have 2 see invis golem you have to get by. You either have to fight them or use trickery. I've done both, but I like to kill them myself. When they're dead, there's less problems that can arise. The rest of the way is littered with undead creatures and live ones. You have to be very careful navigating your way though HS. There's always a surprise lurking around the corner. Sometimes a named can be up so this could be even more problematic.

Places like Chardok are fairly good xp with a burn. But with an EM 25 pet it's very trivial for the most part. So be aware of that.